Multi-step transmissions are used for example in electric drives for driving an electrically driven axle of a motor vehicle in different speed ranges. A multi-step transmission normally comprises a rotatingly drivable input shaft, an output shaft and at least one first transmission stage as well as a second transmission stage. By configuring torque transmitting members of the first and the second transmission stage, torque can be transmitted to the output shaft at different transmission ratios. The electric drive can serve as a single drive for the motor vehicle or it is possible, additionally, to provide an internal combustion engine. In this case, the electric drive and the internal combustion engine can either drive the motor vehicle individually or jointly superimposed.
An electric drive commonly comprises an electric machine as well as a downstream reduction gearing which translates a high speed into a low speed. The reduction gearing can be switchable and is then also referred to as a multi-step transmission. From the reduction gearing the torque is transmitted to the driveline of a motor vehicle, for example to a differential gearing arranged in the torque flow downstream from the reduction gearing. A differential gearing can distribute an introduced torque to two output shafts which have an equalizing effect relative to one another.
From DE 10 2005 022 926 B3 a drive unit for a motor vehicle is known, having an electric motor, a change-speed gearing with at least two gear stages and a differential. The motor drives the differential via an intermediate shaft. For operating the two transmission stages there is provided an actuator assembly which comprises a rotatingly drivable switching roller. The switching roller is used to operate a switching muff which operates two clutches.
WO 2015/149875 A1 proposes an actuator assembly for a clutch in a multi-step transmission. The actuator assembly comprises an electrically driven ball spindle drive. The ball spindle drive can be used, optionally, to transfer the clutch into three transmission positions: a first transmission stage, a second transmission stage and a neutral position.
WO 2012/007031 A1 proposes an electric drive for a motor vehicle, having an electric motor and a transmission unit. The transmission unit comprises, in a series-connected way, a first transmission stage in the form of a spur gear or chain drive, a second transmission stage in the form of a planetary gearing and a differential gearing. By means of a clutch, the sun gear of the planetary gearing can optionally be connected to the hollow gear in a rotationally fixed way or it can be rotationally fixedly supported on a stationary housing part or it can be changed into an idling condition.
From WO 2012/007030 A1 there is known an electric drive for a motor vehicle, having an electric motor and a transmission unit. The transmission unit comprises a planetary gearing and a differential gearing which are arranged coaxially relative to one another. There is provided a clutch which can be transferred into three transmission positions, i.e., two different transmission stages as well as an idling condition.
Electric drives having multi-step transmissions require considerable structural measures for the clutches and the actuator assembly. In addition, strict requirements have to be met by the multi-step transmission and/or the actuator assembly in the case of functional failure occurring under unforeseeable conditions, so-called Fail-Safe functions.